1. Field of the Invention
The present invention relates to a photosensitive resin composition for use as a material for a printed wiring board. More specifically, it relates to a photosensitive resin composition which has excellent resolution and high heat resistance and which gives, as a material, a printed wiring board excellent in heat resistance and electric insulation under moisture absorption taking place for a long period of time.
2. Prior Art of the Invention
In recent years, electronic machines and equipment are desired to have a decreased size and an increased density, and printed wiring boards are also essentially required to have a smaller size, a higher density, a lighter weight and a smaller thickness. For applying a solder resist material onto a substrate, it is so far general practice to use solder resist material which is curable with ultraviolet light, developable with a diluted alkali and excellent in durability against plating and a solvent. However, in build-up boards having fine wirings inside, and the like, solder resists recently cause a popcorn phenomenon in a solder resist/sealing resin interface, or some other problems. That is, the solder resists cause problems with regard to heat resistance. Further, solder resists are also required to have reliability on insulation between fine wiring lines.
For improving electric insulation and heat resistance, there has been developed a resist utilizing a cyanate ester compound such as a BT resin (Japanese Patent Application No. 8-298641). In development with an alkali, the cyanate ester compound causes a remaining resin after the development, which causes a problem in the steps of electroless plating with nickel and electroless plating with gold. With regard to this problem, it is already known that when a cyanate ester compound such as a BT resin, etc., and an epoxy acrylate are allowed to react with each other in advance for improving developability, electric properties and the developability are improved at the same time (Japanese patent Application No. 11-61075).
The above resist is excellent in resist properties. However, it has been found that the above resist has a problem on durability against heat under long-lasting moisture absorption. When a resist is used for the production of semiconductor packages, the resist is required to have durability against heat under long-lasting moisture absorption, and such durability against heat comes to be of great interest.
Under the circumstances, it is an object of the present invention to provide a photosensitive resin composition which is highly heat-resistant, which is excellent in durability against heat and electric insulation reliability under long-lasting moisture absorption, and which is developable with an alkali.
According to the present invention, there is provided a photosensitive resin (II) which is a reaction product formed by reacting a reaction product (I) prepared from a biphenyl epoxy acrylate (a) of the formula (1) and a cyanate ester compound (b) with a polybasic acid anhydride 
wherein each of R1 and R9 is a hydrogen atom or methyl, and n is an integer of 1 or more.
According to the present invention, there is provided a photosensitive resin composition comprising the above photosensitive resin (II) and an epoxy resin (d).
The present inventors have made diligent studies for achieving the above objects. As a result, it has been found that the above objects can be achieved by using a resin obtained by reacting a difunctional epoxy acrylate having a biphenyl skeleton with a cyanate ester compound and reacting the resultant reaction product with a polybasic acid anhydride, and the present invention has been accordingly completed.
That is, the present invention provides a photosensitive resin (II) obtained by reacting a reaction product (I) prepared from a biphenyl epoxy acrylate (a) of the above formula (1) and a cyanate ester compound (b) with a polybasic acid anhydride (c). It is preferred to use an epoxy acrylate (a) of the formula (1) in which, of R1 to R8, each of R2, R3, R6 and R7 is a hydrogen atom and the others are methyl or an epoxy acrylate (a) of the formula (1) in which, of R1 to R8, each of R3 and R7 is a hydrogen atom and the others are methyl.
The present invention also provides a photosensitive resin composition (III) comprising the above photosensitive resin (II) and an epoxy resin (d).
The cyanate ester compound (b) used in the present invention refers to an aromatic organic compound generally having at least 2 but not more than 5 cyanato groups (xe2x80x94Oxe2x80x94Cxe2x89xa1N) per molecule, and the cyanate groups bond directly to an aromatic ring of the aromatic group R.
Specific examples of the cyanate ester compound (b) include 1,3- or 1,4-cyanatebenzene, 1,3,5-tricyanatebenzene, 1,3-, 1,4-, 1,6-, 1,8-, 2,6- or 2,7-cyanatenaphthalene, 1,3,6-tricyanatenaphthalene, 4,4xe2x80x2-dicyanatebiphenyl, bis(4-cyanatephenyl)methane, 2,2-bsi(4-cyanatephenyl)propane, 2,2-bsi(3,5-dibromo-4-cyanatephenyl)propane, bis(4-cycanatephenyl)ether, bis(4-cyanatephenyl)thioether, bis(4-cyanatephenyl)sulfone, tris(4-cyanatephenyl)phosphite, tris(4-cyanatephenyl)phosphate and cyanates obtained by reactions of novolak and cyan halides.
In addition to these, the cyanate ester compound (b) can be also selected from polyfunctional cyanate ester compounds described in Japanese Patent Publications Nos. 41-1928, 43-18468, 44-4791, 45-11712, 46-1112, 47-26853 and 51-63149. These cyanate ester compounds may be used alone or in combination. The content of impurities such as hydrolyzing Cl and Na in these compounds is very small, and when any one of these is incorporated as one component in the present invention, the overall content of impurities decreases, so that the photosensitive resin (II) is suitable as a peripheral material for semiconductors.
Further, there may be used triazine-ring-possessing prepolymers having a molecular weight of 200 to 6,000 formed by trimerization of cyanate groups of the above polyfunctional cyanate ester compounds. The prepolymers can be obtained by polymerizing the above polyfunctional cyanate ester compound monomers in the presence of a catalyst selected from acids such as mineral acids and Lewis acids, bases such as sodium alcoholate and tertiary amines or salts such as sodium carbonate. The prepolymers partially contain monomers and have the form of a mixture of a monomer and a polymer, and these materials are preferably used in the present invention.
The polybasic acid anhydride (c) for use in the present invention refers to an acid anhydride having at least 2 carboxylic acids per molecule. Examples of the polybasic acid anhydride (c) include phthalic acid anhydride, trimellitic acid anhydride, pyromellitic acid anhydride, naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, benzophenonetetracarboxylic acid anhydride, tetrahydrophthalic acid anhydride, hexahydrophthalic acid anhydride, 4-methylhexahydrophthalic acid anhydride, 3-methylhexahydrophthalic acid anhydride, succinic acid anhydride, dodecenylsuccinic acid anhydride, ethylene glycol bias(anhydrotrimellitate), 3,3xe2x80x2,4,4xe2x80x2-diphenylsulfonetetracarboxylic acid dianhydride, 4,4xe2x80x2-oxydiphthalic acid anhydride, 4,4xe2x80x2-diphthalic acid anhydride, and others having molecules containing acid anhydrides. These may be used alone or in combination. A mixture containing pyromellitic acid anhydride is preferred.
The method of practicing the present invention will be explained below.
First, when the epoxy acrylate (a) having a biphenyl skeleton is reacted with the cayante ester compound (b), generally, 5 to 40 part o the cyanate ester compound (b) can be modified per 100 parts of the epoxy acrylate (a), although the amount ratio is not specially limited. When the amount of the cyanate ester compound (b) is small, the heat resistance, the moisture absorption and the electric insulation after a pressure cooker test are insufficient. When the amount of the cyanate ester compound (b) is large, there is caused a problem during the production of a resin, such as gelation during the synthesis of a resin. The amount of the cyanate ester compound (b) is therefore preferably 5 to 30 parts. The reaction temperature is 50 to 100xc2x0 C., and the reaction time period is 5 to 100 hours. A solvent may be used for adjusting a viscosity during the reaction. Although not specially limited, the solvent is preferably selected from those having such a boiling point that almost all of it can be evaporated during the drying of a layer of a photosensitive resin composition. Examples of the solvent include ketones such as acetone and methyl ethyl ketone, esters such as dipropylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether, diethyl ether monomethyl ether acetate and diethyl ether monomethyl ether, and aromatic hydrocarbons such as solvent naphtha, toluene, xylene and ethylbenzene. These solvents may be used alone or in combination.
Then, the resin is modified with a carboxylic acid to render the resin developable with an alkali.
The modification amount is 10 to 90 parts per 100 parts of the epoxy acrylate (a), and there is obtained a resin having a resin acid value of 40 to 400 mgKOH/g. An acid value of approximately 50 to 120 is effective in view of developability, and it is required to add a polybasic acid anhydride in such an amount that the acid value is brought into the above range. In this case, a solvent may be also used for adjusting a viscosity, and the solvent can be selected from the above-described solvents.
The use of the photosensitive resin (II) in the photosensitive resin composition (III) will be explained below. For use as a resist ink, an epoxy resin (d) for improving various properties such as adhesion, heat resistance and soldering durability and a photopolymerization initiator for initiation of polymerization are required in addition to the photosensitive resin (II).
Examples of the epoxy resin (d) used in the present invention include a bisphenol A type epoxy resin, a bisphenol F type epoxy resin, a biphenyl type epoxy resin, a novolak type epoxy resin, triglycidyl isocyanurate and an alicyclic epoxy resin. These epoxy resins are used alone or in combination. Of these, triglycidyl isocyanurate is preferred since a dark reaction takes slowly during the drying of a resist ink.
The amount of the epoxy resin (d) is required to be equivalent to, or greater than, the equivalent weight of carboxylic acids contained in the photosensitive resin, and it is preferably 1 to 1.5 equivalent weights. Further, the amount of the epoxy resin (d) based on the composition is preferably 50 parts or less, particularly preferably 3 to 45 parts.
The photopolymerization initiator used in the present invention is selected from known photopolymerization initiators. Examples of the photopolymerization initiator include xcex1-diketones such as benzyl and diacetyl, acyloin ether, thioxanthones such as thioxanthone and 2,4-diethylthioxanthone, benzophenones such as benzophenone and 4,4xe2x80x2-bis(dimethylamino)benzophenone, acetophenones such as acetophenone, 2,2xe2x80x2-dimethoxy-2-phenylacetophenone and xcex2-methoxyacetophenone, quinines such as anthraquinone and 1,4-naphthoquinone, and peroxides such as di-tert-butylperoxide. These photopolymerization initiators may be used alone or in combination. The amount of the photopolymerization initiator is 0.1 to parts by weight, preferably 0.2 to 10% by weight.
The above components are kneaded by a generally known method in which dispersing can be carried out homogeneously with a three-roll mill, a homogenizer or a bead mill. Further, the photosensitive resin composition (III) may contain known additives such as a filler, a curing promoter, an anti-foamer, a surface-treating agent, a flame retardant, a pigment and a dye. The filler includes silicas such as natural silica, fused silica and amorphous silica, white carbon, titanium white, aerosil, alumina, talc, natural mica, synthetic mica, kaolin, clay, aluminum hydroxide, magnesium hydroxide, zinc molybdate, barium sulfate, E-glass, A-glass, C-glass, L-glass, D-glass, S-glass, M-glass and G20-glass. The curing promoter includes imidazoles such as 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 2-phenylimidazole and 1-benzyl-2-methylimidazole, tertiary amines such as benzyldimethylamine and phosphorus compounds such as phosphine compounds and phosphonium compounds.
A resist ink preferably prepared such that the content of the photosensitive resin (II) as a main component, based on the total amount, of 30 to 90% by weight. When the resin content is less than % by weight, it is difficult to secure the formation of a resin layer having a uniform thickness, and the resist ink is not suitable for use in printed wiring boards since the layer is unstable in quality. When the resin content exceeds 90% by weight, a layer formed of the resist ink on a printed wiring board may peeled off, or the printed wiring board may vary in electric characteristics.
The photosensitive resin composition (III) of the present invention is useful as a resist ink of an etching resist and permanent resists such as a solder resist and an insulating resist for a build-up printed wiring board. Further, it can be used as a coating composition, a coating agent or an adhesive. The photosensitive resin composition (III) of the present invention is cured, for example, by the following method, to give a cured product.
That is, the photosensitive resin composition of the present invention is applied to a printed wiring board by a screen printing method, a spray method, a roll coating method, an electrostatic painting method or a curtain coating method, to form a coating having a thickness of to 160 xcexcm, the coating is dried at 60 to 110xc2x0 C. and then exposed to ultraviolet light through a negative film, an unexposed portion is developed with a diluted alkali aqueous solution, and the coating was further fully cured by irradiation with ultraviolet light or heating for improving its properties, whereby a cured coating is obtained.
The light source for applying active energy for curing the photosensitive resin composition (III) of the present invention includes a low-pressure mercury lamp, an intermediate-pressure mercury lamp, a high-pressure mercury lamp, an ultrahigh-pressure mercury lamp, a xenon lamp and a metal halide lamp. The dose is 100 to 2,000 mJ/cm2, preferably 250 to 1,000 mJ/cm2. When the dose is small, an exposed portion is not cured, and is dissolved during development.
After the photosensitive resin composition (III) of the present invention is exposed to the above light, development can be carried out by a known method such as a spray development method in which a developer solution is applied by spraying or a dipping development method in which a printed wiring board is dipped in a developer solution and vibrated. The temperature of the developer solution is 5 to 50xc2x0 C., preferably 25 to 40xc2x0 C. When the temperature of the developer solution is low, there is caused a problem that development takes a time and that developing performance is poor. When the above temperature is high, a portion cured by exposure comes to be dissolved. The developer solution can be selected from known diluted alkali aqueous solutions such as a sodium hydroxide aqueous solution and an ammonium hydroxide aqueous solution. The content of an alkali agent in the aqueous solution is preferably 0.1 to 5.0% by weight.
When the photosensitive resin composition (III) of the present invention is cured by heating after the development, the curing temperature is 100 to 250xc2x0 C., preferably 120 to 200xc2x0 C. When the curing temperature is low, the curing takes a time. When the curing temperature is high, there is caused a problem of discoloration or partial swelling.